During the disco and funk era, in many venues the dance floor was often located below the seating area, so to "get down" was to go down to the dance floor. Originally meant in reference to dancing, eventually leading to sexual implications. However, it also means "to be cool with".
A most useful term.

Other forms: Get down, gettin down, getting down.
There is a more recent version of the sexual meaning is "going down (town)".

Disco conversation in the 1970s: "Get down with your bad self... go dance, you supafly!"
Intimate conversation: "The she started going down, the sex was so hot, I was blazin"
Casual conversation: "I can get down with that... it's cool"